Process for horizontally fracturing subsurface earth formations

ABSTRACT

Horizontal fractures are formed in a subsurface earth formation which tends to fracture vertically at the naturally occurring formation temperature by a process which includes the steps of extending at least one well borehole into the formation, generating a vertical fracture by pressurizing said borehole, injecting hot fluid into at least one borehole to heat the formation, continuing the injection of hot fluid until thermal stressing of the formation matrix material causes the horizontal compressive stress in the formation to exceed the vertical compressive stress therein at a location selected for a second well, extending the borehole of the second well into the formation, and hydraulically fracturing the formation through this second well borehole to form a horizontal fracture extending therefrom into the formation.

xx 88 00 BB 66 66 11 y at the natuperature by a process which ing atleast one well borehole into y pressurizing ne borehole to eat theformation, continuing the injection of hot fluid until 3,303,883 2/1967Slusser Dionysios M. Phoeas, both of Houston, Tex. 3,455,391 7/1969Matthews et al..... "11,710 3,501,201 3/1970 Closmann et a1. Feb. 16,1970 3,500,913 3/1970 Nordgren et Primary Examinerlan A. Calvert Shell011 Company New York NY. Attorneys-J. H. McCarthy and T. E. BleberABSTRACT: Horizontal fractures are formed in a subsurface earthformation which tends to fracture verticall rally occurring formationtern 166/271, eludes the steps of extendin 1 166/308 the formation,generating a vertical fracture b Ezlb 43/24, said borehole, injectinghot fluid into at least 0 7 166/259, themtal stressing of the formationmatrix material causes the 271, 303, 303 horizontal compressive stressin the formation to exceed the R f cud vertical compressive stresstherein at a location selected for a e "cums second well, extending theborehole of the second well into the fonnation, and hydraulicallyfracturing the formation 166/308 UX through this second well borehole toform a horizontal frac- 166/271 X ture extending therefrom into thefonnation.

UNITED STATES PATENTS PROCESS FOR HORIZONTALLY FRACIURING SUBSURFACEEARTH FORMATIONS 10 Claims, 9 Drawing Figs.

Unlted States Patent [72] Inventors Philip J. Closmann;

[21] AppLNo. [22] Filed [45] Patented Oct. 19,1971 [73] Assignee2,859,818 11/1958 Halletal...................... 3,129,761 4/1964Staadt....

PATENTEUnm 19 I9?! 3. 6 1 3 785 SHEET 1 or 3 3 INVENTORS:

- P. J. CLOSMANN D.M. PHOCAS PAIENTEnnm 19 l97l SHEET 2 BF 3 I 1 Us FIG-llflll Ill 1 Will FIG. 5

INVENTORS P. J. CLOSMANN D. M- PHOCAS FIG. 6

PATENTEDHU 9 I97! SHEET 3 BF 3 Iv. I

FIG. 9

INVENTORS:

J- CLOSMANN D. M. PHOCAS PROCESS FOR HORIZONTALLY FRACTURING SUBSURFACEEARTH FORMATIONS BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates to a process for forming horizontally directedfractures in a subsurface earth formation which in its natural statetends to fracture vertically.

2. Description of the Prior Art Hydraulic fracturing is a conventionalprocedure that is often employed where the permeability of an earthformation is too low to pennit fluid to flow into or out of theformation at a rate which is economically suitable in respect to arecovery of petroleum or other material from the earth formation. Inpetroleum thermal recovery operations, hydraulic fractures are formed topermit heat to be injected over a wide area into the oil-bearingformation. Fracturing is also useful in recovering liquifiablecomponents from essentially impermeable earth formations such as oilshale or bed deposits of cinnabar.

In a hydraulic fracturing operation, a fluid is confined in a region inwhich it is in contact with a subterranean earth for mation and thepressure on the fluid is increased until a fracture is formed within theearth formation. It is generally recognized that hydraulic fracturesform along planes which are perpendicular to the least one of the threeprincipal compressive stresses that exist along a vertical and twomutually perpendicular horizontal axes within any subterranean earthformation. Where the vertical stress is least, hydraulic fracturingproduces a horizontal fracture. In such a situation the fracturingoccurs when the pressure applied to the fracturing fluid exceeds apressure that results from the weight of the overlying earth formationsby an amount sufficient to overcome the tensile and/or shear strength ofthe earth formation, or rock.

The pressure which results from the weight of the overlying earthformations is commonly referred to as the overburden pressure. It isgenerally equal to, or slightly less than, about 1 pound per square inchper foot of depth. The pressure required to cause a failure of asubsurface earth formation in situ is commonly referred to as thefracturing pressure or formation breakdown pressure.

In respect to a horizontal fracture, the fracturing pressure isnecessarily greater than the overburden pressure, since the overburdenmust be lifted in order to separate the layers of rock. In respect to avertical fracture, the vertical compressive stress is greater than oneor both of the principal compressive stresses that are perpendicular toeach other within the horizontal plane. In the latter type of situation,the hydraulic fractures are vertical, and are oriented in a planeperpendicular to the weaker of the two horizontal principal compressivestresses.

Where a fracture is to be used in an oil-producing or fluidminingoperation, it is generally advantageous to use a horizontal fracture. Ahorizontal fracture is better than a vertical fracture in respect toboth the distributing of fluid over a region having a significant arealextent, and the interconnecting of a pair of wells. However, it hasproven to be difficult to overcome the fracturing tendencies that aredictated by regional tectonics, and in many of the reservoirs in theUnited States the least principal stress is horizontal. In suchreservoirs, in order to form a horizontal fracture, it is necessary toeither increase the horizontal compressive stresses or decrease thevertical compressive stress, or do both, until the vertical stressbecomes the least of the three principal stresses.

Since a confined material can be stressed by heat, the state of stressthat exists in the rocks within a subterranean region can be changed byheating a subterranean zone of the appropriate size and shape. Theresulting stress distribution is a function of temperature distribution,the elastic constants of the material, the coefficient of thermalexpansion of the material, and the boundary conditions. One such methodfor thermally increasing the horizontal compressive stresses until thevertical stress becomes the least principal stress is taught by C. S.Matthews, P. Van Muers and C. W. Volek in US. Pat.

No. 3,455,39l. Therein, a well is fractured at the earth formationtemperature and hot liquid is flowed into a resulting vertical fractureto heat the earth formation around the fracture. As the formation isheated it thermally expands and the walls of the fracture move towardeach other until the fracture is closed. At this point, the rocks areleft with unrelieved components of thermally induced horizontal stress.As the fracture becomes closed, the pressure required to cause fluid toflow into the rocks becomes higher and, when a second fracturingpressure is reached, another fracture is formed. Such a second fractureis apt to be another vertical fracture, and one that forms in adirection perpendicular to the first fracture at a pressure which isless than the overburden pressure. By continuing the injection of hotliquid, the above sequence of events is repeated and the injectionpressure is further increased. Eventually, the pressure required toinject liquid exceeds the overburden pressure and a horizontal fractureis formed.

Tests and theory indicate that heating the face of a vertical fracturein an earth formation generally causes the vertical stress at the heatedface to increase at a rate at least as great as the rate of increase ofthe horizontal stresses at that face; whereas, the horizontal stressesat some distance away from the heated surface increase more rapidly'thanthe vertical stress. The present invention provides a method for takingadvantage of the differential rate of stress increase by heating theformation at one well bore and then fracturing the formation throughanother well bore at some distance from the first.

BRIEF SUMMARY OF THE INVENTION Horizontally directed fractures areformed in a subterranean earth formation that tends to fracturevertically at the naturally occurring formation temperature by a methodwhich comprises the steps of extending at least one well borehole intothe formation, preferably fracturing the formation to form verticallydirected fractures which extend into the formation from the borehole,injecting a hot fluid having a temperature significantly higher than thesurrounding earth formation, continuing the injection of hot fluid untilthermal stressing of the formation matrix material causes the horizontalcompressive stress in the formation to exceed the vertical compressivestress therein at a location selected for a second well, extending theborehole of this second well into the formation (if the borehole was notpreviously so extended), and hydraulically fracturing the formationthrough this second well borehole to form a horizontal fractureextending therefrom into the formation.

The first well is preferably located on the basis of knowledge of thetrend along which vertical fractures are likely to be formed. Thelocation of the second well may be selected by solving therrnoelasticequations to determine the compressive stress distribution in thereservoir rock after a planned period of heating. The second well ispreferably placed at a location where the amount by which the horizontalcompressive stress exceeds the vertical compressive stres is a minimum.If a specific location is desired for the second well, the thermoelasticequations may be solved to find the period of heating required to makethe vertical compressive stres the least principal stress at the desiredlocation. The second well may be extended into the formation of interestbefore or after heating has been initiated in the first well. However,the second well should not be fractured until heating of the first wellhas been continued for a period of time sufficient to reverse therelative magnitude of the horizontal and compressive stresses in theformation at the location of the second well.

In a modification of the proces two well boreholes may be extended intothe subsurface formation which is then fractured through each well atthe naturally occurring formation temperature to form at least a pair ofparallel vertically directed fractures, one of the pair extending intothe formation from each borehole. The fonnation is heated by injectinghot fluid into both of these parallel fractures for a time sufficient toreverse thermally the relative sizes of the horizontal and verticalcompressive stresses in the reservoir at one or more locations (betweenthe pair of parallel fractures) selected for one or more additionalwells.- The necessary period of hot fluid injection may be determined bysolving the thermoelastic equations to determine the heating periodrequired to reverse thermally the relative sizes of the vertical andhorizontal compressive stresses at a given location. When heating hascontinued for a period sufficient to make the vertical compressivestress the least principal stress at a selected location the formationmay be hydraulically fractured through a well bore extending into theformation at that selected location to form a horizontal fracture. As inthe two well case, the well bore at the selected location may beextended into the formation before or after heating is initiated in thefirst fractured two wells, however, the well at the selected locationshould not be fractured before the relative sizes of the vertical andhorizontal compressive stresses in the formation are thermally reversedat that selected location.

If in the above modification of the process, the heating of the firstfractured two wells from which the pair of parallel vertically directedfractures extends is continued for a sufficient time, the verticalcompressive stress may become the least principal stress in theformation at the well bore of one or both of these two wells. Therefore,in a further modification of the process, the formation may behydraulically fractured through one or both of the first fractured twowells after such a period of heating to form horizontal fracturesextending from the borehole of the hydraulically fractured well into theearth formation.

In any of the above methods of practicing this invention, well bores inthe subsurface earth formation may be interconnected by extending thefractures formed from well bores at the location of which the relativesizes of the horizontal and vertical compressive stresses have beenthermally reversed until fluid communication with adjacent wells isachieved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional viewshowing a completed well, a proposed well, and a surrounding earthformation.

FIG. 2 is a horizontal cross section taken along line 2-2 of FIG. 1which shows the formation of interest after it has been treated inaccordance with a process of the present invention.

FIG. 4 is a vertical cross section of the formation of interest takenalong the line 4-4 of FIG. 3 after the formation has been treatedaccording to a process of this invention.

FIG. 5 is a cross-sectional view showing two completed wells, a proposedwell, and a surrounding earth formation.

FIG. 6 is a horizontal cross section taken along line 6-6 of FIG. 5which shows the formation of interest after it has been treated with aprocess of this invention.

FIG. 7 is a plot of stress versus distance along the line 88 of FIG. 6after a period of heating from wells 12 and 13.

FIG. 8 is a vertical cross section of the formation of interest takenalong the line 8-8 of FIG. 6 after the formation has been treatedaccording to an embodiment of this invention.

FIG. 9 is a plot of stress versus distance along the line 8-8 of FIG. 6after a long period of heating from wells 12 and 13.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a well which isprovided with equipment suitable for use in practicing this invention. Astring of casing 14 is installed to extend from a surface location intoan earth formation 15 in which it is desired to create a horizontalfracture. The casing is provided with perforations 16 which open thewell into fluid communication with the earth formation 15. A tubingstring 17 carrying a packer 18 provides a means of conveying fluid froma surface location to a location adjacent to earth formation 15. Theupper end of the tubing string is connected to a pump 19 which may beconnected to heating and fluid-handling units (not shown) which unitscan be conventional types of such equipment.

In practicing one embodiment of this invention, the borehole of thefirst well 10 of a pair of wells 10 and I1 is extended into the earthformation I5 and the well is equipped as described above. This well 10is preferably located on the basis of knowledge of the trend along whichvertical fractures are likely to be fonned. Such knowledge is generallyavailable from measurements of fracture orientation in other wells orfrom information on the regional tectonics. A preferred placement of thefirst well 10 is such that a line connecting this well and the proposedlocation of the second well 11 of the pair of wells is parallel to thedirection 26 of the naturally occurring least principal stress in theearth formation 15.

Earth formation 15 is preferably fractured through well 10 at thenaturally occurring formation temperature by a conventional means suchas pumping liquid through the tubing 17 to a selected region of thecasing 14 which is within the earth formation 15. In this region theliquid is confined by the packer l8 and the borehole bottom 29 and isconveyed by the perforations 16 into contact with the earth formation15. The pressure is increased until the bottom hole liquid pressureexceeds the fracturing pressure and causes a fracture, such as fracture31 (FIG. 2), to form in the earth formation. This fracture, most likely,is vertically oriented and extends from the well 10 in a directionperpendicular to the direction 26 of the naturally occurring leastprincipal stress. This fracture is preferably propped open with apropping material 35 by procedures known in the art.

Hot fluid such as hot water or steam may then be flowed down tubing 17into the well 10 and the fracture 31. This fluid should have atemperature greater than that of the surrounding earth formation. Theinjection should be continued for a time sufiicient to reverse thermallythe relative sizes of the horizontal and vertical compressive stresseswithin the earth formation at a location selected for the second well13. That is, hot fluid injection is continued until thermal stressing ofthe matrix material of the earth formation 15 causes the horizontalcompressive stress to be greater than the vertical compressive stress atthe location proposed for well 1 1.

At the time such a stress configuration is achieved in the earthformation 15 at the desired location, the borehole of well 11 may beextended into the earth formation 15 and the well 11 equipped in amanner similar to well 10. However, it should be noted that it is notcritical to the practice of this invention that well 11 be extended onlyat this time; the formation 15 may be penetrated with well I1 before orafter heating through the first well 10 is commenced.

Finally, the earth formation is fractured through well 11 by aconventional hydraulic fracturing procedure such as that described withrespect to well 10. The result of this fracturing procedure may be asubstantially horizontal fracture 30 (FIG. 4) extending from the well 11into the earth formation 15. This fracture may be propped with apropping material 37 by procedures known in the art.

The required temperature of the hot fluid injected into fracture 31 andthe extent of time injection must be continued are related to thethermal conductivity, expansion properties, and preexisting stressconditions of earth formation 15. The compressive stress distribution inthe earth formation for a given period of heating from fracture 31 witha fluid having a given temperature may be determined by mathematicalmodeling. For example, for heating from a vertical fracture 31 in atransversely isotropic formation 15 a model may be constructed using thefollowing assumptions:

1. The linear theory of thermoelasticity, involving only infinitesimaldisplacements, is applicable over the temperature and stress conditionsto be encountered. Under these conditions the stresses due to the weightof the overburden are known and may be subtracted from the stressdistribution calculated in the presence of overburden to obtain stressconditions independent of depth.

2. The elastic and thermal constants are not functions of temperature.

3. Temperature and deformations are independent of each other.

4. The temperature distribution within a plane normal to the fracture 31may be computed independently of the temperature distribution along thefracture 31 once the temperature at the fracture face is known.

5. The elasticity problem may be treated as one of plane strain, i.e.,no strain in the horizontal'direction in which the fracture 31 wasoriginally propagated.

6. At sufficiently great distances from the fracture 31 the stresses,displacements, and temperature in the formation are undisturbed by theheating from the fracture.

7. The plane of transverse isotropy corresponds to horizontal beddingplanes of the formation, and the plane of the fracture 31 is parallel toan axis of symmetry, perpendicular to this plane of isotropy.

8. Total stresses can be computed by adding those due to a uniform bodyforce to the thermal stresses. Tectonic stresses, if known, can be addedto the solution obtained.

If we take an xyz coordinate system having the x-y plane as thehorizontal plane of isotropy, the x-direction perpendicular to the planeof the fracture, and the z-axis as an axis of symmetry parallel to theplane of the fracture, then the ther moelastic strain equations are asfollows:

a 1 X y)%-n+a( r) (1) 1 Z= .-W.)%- .+a TTI (2) 1 a 1) 6w WWW 4) 1 bu Ow,7 afia 5 1 .2(1+v) O z Q u E a 0a: (6)

where u=displacement in the x-direction v=displacement in they-direction w=displacement in the Z-direction normal strain component inthe X-direction 3; normal strain component in the y-direction i normalstrain component in the z-direction I Tr, total normal stress inx-direction a, total normal stress in y-direction 0- total normal stressin z-direction r,,= shear stress in y-z plane 1,, shear stress in z-xplane 1' shear stress in x-y plane T= temperature T,.= initial fonnationtemperature E Youngs modulus in plane of isotropy E Young's modulusnormal to plane of isotropy v Poissons ratio for transverse reduction inthe plane of isotropy for tension in the same plane.

v Poissons ratio for transverse reduction in the plane of 55 where pbody force due to the weight of the overburden. From equations (1)through (8), and the assumption of plane strain in the y-direction,i.e., 511/ Oy=0, the following differential equations for displacementsin the x-y plane may be obtained:

T=T (x,s) temperature distribution obtained by solving the heatconduction equation:

a T b T O T x=W+ i'o Pf f a t where K thermal conductivity parallel toplane of isotropy,

K thennal conductivity normal to plane of isotropy, and

p C volumetric heat capacity of fonnation.

Equations (9) and l0) can be solved for an appropriate set of boundaryconditions by numerical methods known in the art such as the finitedifference method. The results of such a solution may be used to computethe compressive stresses in the formation for a given temperaturedistribution from the following relationships:

FIG. 3 shows a stress distribution in the earth formation 15 after aperiod of heating from a single vertical fracture 3!. There is a region33 near the walls of the fracture where the vertical compressive stress0,, exceeds the horizontal compressive stress, 0-,. However, as onemoves away from the fracture face, the vertical stress decreases morerapidly than the horizontal stress and eventually becomes the smallerstress throughout a zone 34 in which thermal stressing of the formationmatrix material has created conditions favorable to the formation ofhorizontal fractures. The borehole of the second well 11 is preferablylocated near the midpoint of this zone 34 so that the horizontalfracture 30 formed fromthe well 11 has maximum arcal extent. For a giventemperature of injected fluid, the heating time required to create aproperly positioned zone 34 in which the vertical compressive stress isthe least principal stress may be determined by solving equations (16)through 18) for various heating times and selecting that heating timewhich will create a zone of thermal stress reversal in which the amountby which the horizontal compressive stress exceeds the verticalcompressive stress is a maximum at the desired location of the well boreof the second well 11.

In a second embodiment of this invention the boreholes of at least twowells such as boreholes of wells 12 and 13 in FIG. may be extended intothe earth formation 15 and equipped in a manner similar to thatheretofore described for well 10. These wells are preferably located sothat a straight line drawn between the two wells is parallel to thedirection 26 of the naturally occurring least principal stress in theearth formation 15. The earth formation 15 may be fractured through eachof the wells 12 and 13 at the naturally occurring formation temperatureby a conventional means. The resulting fractures 33 and 32 (FIG. 6) aremost likely substantially parallel vertically oriented fractures, one ofwhich extends from each of the respective wells 12 and 13 in a directionperpendicular to the direction 26 of the naturally occurring leastprincipal stress. These fractures are preferably propped with a proppingmaterial 36 and 38 by procedures well known in the art. Hot fluid suchas water or steam may then be flowed down the tubing 22 and 23 of wells12 and 13 and into the propped fractures 33 and 32. The injection of hotfluid is continued until thermal stressing of the matrix material ofearth formation 15 causes the vertical compressive stress to become theleast compressive stress throughout a zone 39 (FIG. 7) between theparallel fractures 33 and 32 which zone includes the proposed locationof one or more additional wells such as well 20. These additional welllocations are preferably midway between fractures 31 and 32. Theborehole of an additional well may then be extended into the earthformation 15 (if it has not previously been so extended) and equipped ina manner similar to that described above with respect to well 10. Theearth formation 15 may then be fractured through this well 20 by aconventional method to form a substantially horizontal fracture 24 (FIG.8) extending from the well 20 into the formation. This fracture 24 ispreferably propped with a propping material 25 by a procedure well knownin the art.

In a modification of this second embodiment the injection of hot fluidthrough the tubing 22 and 23 of the wells 12 and 13 and into contactwith the subsurface earth formation 15 at the faces of the verticalfractures 33 and 32 may be continued until thermal stressing of thematrix material of earth formation 15 causes the vertical compressivestress to become the least compressive stress at the boreholes of thewells 12 and 13 into which hot fluid is being injected (FIG. 9). Whensuch a stress configuration is achieved, the subsurface earth formation15 may be hydraulically fractured through well 12 or well 13 byconventional procedures, preferably with a heated liquid, to form asubstantially horizontal fracture (not shown) extending from theborehole of the well into the formation. This substantially horizontalfracture may be propped by procedures known in the art.

In any of the above methods of practicing this invention, boreholes inthe subsurface earth formation may be interconnected by extending thefractures formed from a well at the location of which the relative sizesof the horizontal and vertical compressive stresses have been thermallyreversed until fluid communication with one or more adjacent wells isachieved.

In summary, this invention provides a process for forming substantiallyhorizontal fractures in a subsurface earth formation in which fractureswhen formed at the naturally occurring formation temperature tend to bevertically directed. The

process comprises: extending into the subsurface earth formation atleast a first well of a group of at least two wells, injecting throughthe first well and into contact with the subsurface earth formation aheated fluid having a temperature greater than that of the surroundingearth formation, continuing said injection of heated fluid for a timesuflicient to heat the subsurface earth formation and thereby cause thevertical compressive stress to become the least principal stress withinthe subsurface earth formation at a location selected for a second wellof the group of at least two wells, extending into the heated subsurfaceearth formation at said location a second well of the group of at leasttwo wells, and hydraulically fracturing the heated subsurface earthformation at the location of said second well to form a substantiallyhorizontal fracture extending from the second well into the subsurfaceearth formation. Prior to the step of injecting a hot fluid through thefirst well and into contact with the subsurface earth formation saidsubsurface earth formation may be fractured at the location of the firstwell to form a substantially vertically directed fracture extending fromsaid first well into the subsurface earth formation. At least some ofthe wells in the group of wells may be interconnected by extending thesubstantially horizontal fractures formed in the subsurface earthformation at the location of the second well until fluid communicationwith at least one adjacent well is achieved.

To form substantially horizontal fractures in a subsurface earthformation penetrated by a plurality of wells in which formationfractures fonned at the naturally occurring formation temperature tendto be vertically directed the invention provides a modified processwhich comprises: injecting through at least one well and into contactwith the subsurface earth formation a heated fluid having a temperaturegreater than the temperature of the surrounding earth formation,continuing said injection of heated fluid for a time sufficient to heatthe subsurface earth formation and thereby cause the verticalcompressive stress to become the least principal at at least oneselected location in the subsurface earth formation at which it isdesired to form a horizontal fracture, providing another well which wellpenetrates the heated subsurface earth formation at said selectedlocation, and hydraulically fracturing the heated subsurface earthformation at the selected location to form a substantially horizontalfracture extending into the subsurface earth formation.

In one embodiment of this modified process, prior to the step ofinjecting a heated fluid through at least one well and into contact withthe subsurface earth location, the subsurface earth formation may befractured at the location of at least some of the wells through whichsaid hot fluid is injected.

In a preferred embodiment of the modified process the subsurface earthformation is fractured prior to the step of injecting a heated fluid atthe location of at least two wells through which wells said hot fluid isthen injected. Preferably, at least some of the fractures formed whensaid wells are fractured prior to the step of injecting a heated fluidare substantially vertically directed fractures at least one fracture ofwhich extends from each of said wells in a direction such thatsubstantially parallel fractures are formed.

When practicing this invention according to this preferred embodiment ofthe modified process, at least one selected location in the subsurfaceearth formation at which it is desired to form a horizontal fracture maybe the location of one of the two wells through which the hot fluid isinjected.

In practicing this invention according to any of the above embodimentsof the modified process at least some wells of the plurality of wellsmay be interconnected by extending at least some of the substantiallyhorizontal fractures formed in the subsurface earth formation from wellsat the selected locations until fluid communication with adjacent wellsis achieved.

We claim as our invention:

1. A process for forming substantially horizontal fractures in asubsurface earth formation in which fractures when formed at thenaturally occurring formation temperature tend to be verticallydirected, which process comprises:

extending into the subsurface earth formation at least a first well of agroup of at least two wells;

injecting through the first well and into contact with the subsurfaceearth formation a heated fluid having a temperature greater than that ofthe surrounding earth formation;

continuing said injection of heated fluid for a time sufficient to heatthe subsurface earth formation and thereby cause the verticalcompressive stress to become the least principal stress within thesubsurface earth formation at a location selected for a second well ofthe group of at least two wells;

extending into the heated subsurface earth formation at said location asecond well of the group of at least two wells; and

hydraulically fracturing the heated subsurface earth formation at thelocation of said second well by injecting fluid into said second welland increasing the pressure on said fluid to form a substantiallyhorizontal fracture extending from the second well into the subsurfaceearth formation.

2. The process of claim I wherein prior to the step of injecting a hotfluid through the first well and into contact with the subsurface earthformation said subsurface earth formation is fractured at the locationof the first well to form a substantially vertically directed fractureextending from said first well into the subsurface earth formation.

3. The process of claim 1 wherein at least some of the wells in thegroups of wells are interconnected by extending the horizontal fracturesformed in the subsurface earth formation at the location of the secondwell until fluid communication with at least one adjacent well isachieved.

4. A process for forming horizontal fractures in a subsurface earthformation penetrated by a plurality of wells in which formationfractures formed at the naturally occurring formation temperature tendto be vertically directed, which process comprises: 1

injecting through at least two wells and into contact with thesubsurface earth formation a heated fluid having a temperature greaterthan the temperature of the surrounding earth formation;

continuing said injection of heated fluid for a time sufficient to heatthe subsurface earth formation and thereby cause the verticalcompressive stress to become the least principal stress within the earthfonnation at at least one selected location in the subsurface earthformation at which it is desired to form a horizontal fracture;

providing a well penetrating the heated subsurface earth formation atsaid selected location; and

hydraulically fracturing the heated subsurface earth formation at theselected location by injecting fluid into the well penetrating thesubsurface earth formation at the selected location and increasing thepressure on said fluid to form a substantially horizontal fractureextending into the subsurface earth formation.

5. The process of claim 4 wherein prior to the step of injecting aheated fluid through at least one well and into contact with thesubsurface earth formation said subsurface earth location is fracturedat the location of at least some of the wells through which said hotfluid is injected.

6. The process of claim 5 wherein prior to the step of injecting aheated fluid, the subsurface earth formation is fractured at thelocation of at least two wells through which said hot fluid is theninjected.

7. The process of claim 6 wherein at least some of the fractures formedwhen said two wells are fractured prior to the step of injecting aheated fluid are substantially vertically directed fractures at leastone fracture of which extends from each of said wells in a directionsuch that substantially parallel fractures are formed, one of saidparallel fractures extending from each of said wells.

8. The process of claim 7 wherein at least one selected location in thesubsurface earth formation at which it is desired to form a horizontalfracture is a location between said substantially1parallel fractures.

9. he process of claim 6 wherein at least one selected location in thesubsurface earth formation at which it is desired to form a horizontalfracture is the location of one of the at least two wells through whichthe hot fluid is injected.

10. The process of claim 4 wherein at least some of the plurality ofwells are interconnected by extending at least some of the horizontalfractures formed in the subsurface earth formation from wells at theselected locations until fluid communication with adjacent wells isachieved.

1. A process for forming substantially horizontal fractures in asubsurface earth formation in which fractures when formed at thenaturally occurring formation temperature tend to be verticallydirected, which process comprises: extending into the subsurface earthformation at least a first well of a group of at least two wells;injecting through the first well and into contact with the subsurfaceearth formation a heated fluid having a temperature greater than that ofthe surrounding earth formation; continuing said injection of heatedfluid for a time sufficient to heat the subsurface earth formation andthereby cause the vertical compressive stress to become the leastprincipal stress within the subsurface earth formation at a Locationselected for a second well of the group of at least two wells; extendinginto the heated subsurface earth formation at said location a secondwell of the group of at least two wells; and hydraulically fracturingthe heated subsurface earth formation at the location of said secondwell by injecting fluid into said second well and increasing thepressure on said fluid to form a substantially horizontal fractureextending from the second well into the subsurface earth formation. 2.The process of claim 1 wherein prior to the step of injecting a hotfluid through the first well and into contact with the subsurface earthformation said subsurface earth formation is fractured at the locationof the first well to form a substantially vertically directed fractureextending from said first well into the subsurface earth formation. 3.The process of claim 1 wherein at least some of the wells in the groupsof wells are interconnected by extending the horizontal fractures formedin the subsurface earth formation at the location of the second welluntil fluid communication with at least one adjacent well is achieved.4. A process for forming horizontal fractures in a subsurface earthformation penetrated by a plurality of wells in which formationfractures formed at the naturally occurring formation temperature tendto be vertically directed, which process comprises: injecting through atleast two wells and into contact with the subsurface earth formation aheated fluid having a temperature greater than the temperature of thesurrounding earth formation; continuing said injection of heated fluidfor a time sufficient to heat the subsurface earth formation and therebycause the vertical compressive stress to become the least principalstress within the earth formation at at least one selected location inthe subsurface earth formation at which it is desired to form ahorizontal fracture; providing a well penetrating the heated subsurfaceearth formation at said selected location; and hydraulically fracturingthe heated subsurface earth formation at the selected location byinjecting fluid into the well penetrating the subsurface earth formationat the selected location and increasing the pressure on said fluid toform a substantially horizontal fracture extending into the subsurfaceearth formation.
 5. The process of claim 4 wherein prior to the step ofinjecting a heated fluid through at least one well and into contact withthe subsurface earth formation said subsurface earth location isfractured at the location of at least some of the wells through whichsaid hot fluid is injected.
 6. The process of claim 5 wherein prior tothe step of injecting a heated fluid, the subsurface earth formation isfractured at the location of at least two wells through which said hotfluid is then injected.
 7. The process of claim 6 wherein at least someof the fractures formed when said two wells are fractured prior to thestep of injecting a heated fluid are substantially vertically directedfractures at least one fracture of which extends from each of said wellsin a direction such that substantially parallel fractures are formed,one of said parallel fractures extending from each of said wells.
 8. Theprocess of claim 7 wherein at least one selected location in thesubsurface earth formation at which it is desired to form a horizontalfracture is a location between said substantially parallel fractures. 9.The process of claim 6 wherein at least one selected location in thesubsurface earth formation at which it is desired to form a horizontalfracture is the location of one of the at least two wells through whichthe hot fluid is injected.
 10. The process of claim 4 wherein at leastsome of the plurality of wells are interconnected by extending at leastsome of the horizontal fractures formed in the subsurface earthformation from wells at the selected locations until fluid communicationwith adjacent wells is achieved.